This invention relates to the catalytic reduction of carbon oxide with hydrogen and is more particularly concerned with octane improvement of synthetic gasoline resulting from the reaction of carbon oxide with hydrogen. The present invention further relates to the reaction of low octane, principally olefinic synthetic gasoline when it is contacted with solid phosphoric acid catalyst.
A production system of some interest today is the production of synthesis gas (carbon monoxide + hydrogen) from low grade fuels such as coal, and the synthesis of hydrocarbon constituents useful as motor fuels by the Fischer-Tropsch method. Synthesis gas may be obtained from coal, shale oil, petroleum, hydrocarbon gases, municipal refuse, or other carbonaceous material by reaction with steam over a catalyst or by partial direct oxidation using oxygen. For example, methane and steam react over a catalyst at a temperature of about 1400.degree. F. to about 1500.degree. F. in accordance with the reaction: EQU CH.sub.4 + H.sub.2 O.fwdarw.CO + 3 H.sub.2
whereas methane may be reacted directly with oxygen by the reaction: EQU CH.sub.4 + 1/2 O.sub.2 .fwdarw.CO + 2 H.sub.2
Processes in which these reactions are conducted are in widespread use today, particularly in the manufacture of hydrogen. Mostly natural gas and naphtha are utilized as feedstocks in hydrogen manufacturing plants, but of growing interest in view of the present relatively high cost of petroleum and growing scarcity of natural gas, is the utilization of low grade fuels such as coal to produce a carbon oxide plus hydrogen gas mixture from which gasoline may be synthesized.
The Fischer-Tropsch reactions in which hydrocarbons are synthesized from mixtures of carbon oxide and hydrogen are well-known in the art. In addition to gasoline and diesel oil, carbon dioxide, steam, and oxygenated compounds such as acids, alcohols, aldehydes, and ketones are produced by the Fischer-Tropsch reaction. The primary reaction is: EQU n CO + 2n H.sub.2 .fwdarw.(CH.sub.2).sub.n + n H.sub.2 O
this occurs in the temperature range 400.degree. to 670.degree. F., which is sufficiently high for the water-gas shift to take place in the presence of the catalyst: EQU CO + H.sub.2 O.revreaction.CO.sub.2 + H.sub.2
and the overall reaction may be written: EQU n CO + n H.sub.2 .fwdarw.(CH.sub.2).sub.n + n CO.sub.2
by judicious control of the process operating variables including choice of catalyst, one may produce more or less of the above mentioned products. For example, in U.S. Pat. No. 2,510,096, a procedure is disclosed to provide an operation minimizing the effect of the water-gas shift reaction and to reduce if not eliminate production of CO.sub.2 by regulating the CO.sub.2 content of the reactant gas stream supplied to the catalyst at a value of at least 15 percent CO.sub.2.
Several catalysts have been utilized in the Fischer-Tropsch reaction, but the ones of principal commercial value are those activated by cobalt and those activated by iron. Fischer-Tropsch synthesis of gasoline was conducted extensively in Germany during World War II utilizing a catalyst containing five parts thoria, eight parts magnesia, 100 parts cobalt, and 200 parts kieselguhr. Operating conditions included a temperature of about 400.degree. to 430.degree. F. and pressure of about 1 to 15 atmospheres. Most recent operations have utilized predominantly an iron-catalyzed process operating at somewhat higher temperatures of about 550.degree. F. to 700.degree. F. and pressure of about 20 to 40 atmospheres. For production of hydrocarbons predominantly in the gasoline boiling range, a catalyst temperature of about 600.degree. to 650.degree. F. is usually requisite with an iron catalyst. Hydrocarbon products of both catalyst systemes are hydrogen deficient with olefins being the predominant product, especially in the iron-catalyzed process. As reported by Bruner, "Ind. Eng. Chem.," 42:2511 (1949), the following product distributions can be expected with these catalysts:
______________________________________ COBALT CATALYST IRON CATALYST Olefin Olefin Wt.% of Content, wt.% of Content Total Vol.% Total Vol.% ______________________________________ C.sub.3 + C.sub.4 10 40 32 82 Naphtha 30 26 56 85-90 Diesel Fuel 33 8 8 75-85 Residue 27 -- 4 -- ______________________________________
In addition to the generally more favorable product distribution of the iron catalyst, it has an additional advantage of being more flexible in regard to permissible hydrogen-carbon monoxide ratios. Whereas the cobalt promoted catalyst is somewhat limited to the stoichiometric hydrogen-carbon monoxide ratio of 1 to 2 for optimum results, suppression of carbon dioxide formation can be achieved whenever the iron catalyst is utilized by increasing this ratio, which is obtained by suitable recycling of hydrogen gas from the reaction zone effluent to the reaction zone inlet. Accordingly, it is advantageous to use feed gases which contain at least a 3 to 1 ratio of hydrogen to carbon monoxide, and preferably a ratio of 4 to 1 and higher.
A simplified flow scheme of a Fischer-Tropsch process known in the art includes a reaction zone wherein a powdered synthesis catalyst of the iron type is maintained in a state of dense phase fluidization by an upward flowing reactants stream with provision for rapid removal of the heat of reaction. The resulting reaction zone effluent is cooled, condensed, and separated to form a normally gaseous stream consisting of carbon dioxide, hydrogen, light gaseous hydrocarbons, and any unreacted carbon monoxide; a liquid hydrocarbon stream; and an aqueous stream. In addition to the hydrocarbon products, a broad range of oxygenated organic compounds are produced by the Fischer-Tropsch reaction. Generally speaking, the lower molecular weight oxygenated compounds of one to four carbon atoms, especially the alcohols, acids, and aldehydes, are recovered predominantly in the aqueous stream with only a small amount being present in the liquid hydrocarbons stream. While the lower molecular weight oxygenated compounds present in the liquid hydrocarbons stream may in general be recovered by water washing the hydrocarbons stream, considerable percentages of the propyl and higher alcohols are recovered in the liquid hydrocarbons stream. After water washing of the liquid hydrocarbon stream resulting from the synthesis operation, the oxygen content of that stream is about 0.1 weight percent to about 10 weight percent. The liquid hydrocarbons stream is generally further separated to produce a C.sub.3 + C.sub.4 fraction, a synthetic gasoline fraction, and a diesel fraction. The C.sub.3 + C.sub.4 fraction, being principally olefinic, may be further processed over a polymerization catalyst to result in a high octane gasoline product. The synthetic fraction, although characterized by a relatively low research octane number of about 62 to 65, nevertheless has generally been utilized as a gasoline blending component due to the difficulty in further processing the oxygenated compounds.
Bruner, "Ind. Eng. Chem.," 41:2511 (1949) shows the structure of the C.sub.6 to C.sub.8 hydrocarbons in a synthetic gasoline from an iron-catalyzed reaction as follows:
______________________________________ C.sub.6 C.sub.7 C.sub.8 ______________________________________ n-hydrocarbons, wt. percent 75.9 60.2 55.4 monomethyl isomers, wt. percent 20.0 29.3 36.6 dimethyl isomers, wt. percent 0.4 1.7 2.4 cyclic isomers, wt. percent 4.7 8.8 5.6 ______________________________________
It may therefore be concluded that the low octane number of the synthetic gasoline is due to a relatively high straight-chain olefin content. In the case of a synthetic gasoline from a cobalt-catalyzed reaction, it is observed that the normal hydrocarbon content of the C.sub.6 to C.sub.8 fraction is about 15 to 20 weight percent higher than that of an iron-catalyzed synthetic gasoline, resulting in an even lower octane number.
Because of the relatively poor quality of a synthetic gasoline as a gasoline blending component, attempts have been made to isomerize the straight-chain olefins to higher octane branch-chain olefins. Various catalysts have been suggested in the art including an iron promoted catalyst, magnesium chromate/silica alumina, alumina activated with hydrochloric acid, activated clay, alumina activated with hydrogen fluoride, and others.